A variety of communication networks comprising a plurality of base stations with which mobile, portable user equipment can communicate, using radio signals, are known. They include, for example, the GSM network, and the 3rd Generation (3G) network (UTRAN). These networks are typically arranged to provide connectivity between the mobile user equipment (UE) and a core network, such that the user equipment can communicate with other user equipment or indeed other devices at different locations. These communication networks may also be referred to as cellular networks, with different base stations being arranged to provide network coverage (i.e. provide radio communication with the UE) in different areas, known as cells. The coverage of adjacent base stations is typically arranged to overlap, so that there is no loss in network connection as UE moves from one area to another. Clearly, as UE moves within the area covered by such a network it may become necessary for communication that was previously with one base station to be handed over to another base station. The term “base station” in this specification is being used in a broad sense; it is not intended to be limited to the radio transceivers of any particular communications network. It simply refers to a station (or device) of the network which is arranged to transmit radio signals to, and receive radio signals from the UE and so provide connection between the UE and the networks. The base station may also be referred to as a radio transceiver, and base stations include, for example, the base stations of the type used in GSM systems, and the base stations of the UTRA and E-UTRA networks, which are commonly referred to as node Bs (NBs) and enhanced-node Bs (eNBs) respectively.
In certain communications systems, for example those using UTRA or E-UTRA networks, the communication between the user equipment and the base station in a particular area or cell is by radio signals in (i.e. according to) a predefined radio frame format, that format comprising a sequence of radio frames, each having the same length, and the frames of the sequence being numbered sequentially with a System Frame Number (SFN). The frames typically have a defined structure, and this structure may vary with SFN. In such systems, for the UE to be able to communicate with a particular base station it needs to synchronise with the radio frame format being used by that base station, so that, for example, when the UE sends a radio signal to the base station it does so in an appropriate time slot according to the frame format. Typically, the UE will perform this synchronisation using signals received from the base station; it can detect frame edges/boundaries, and will read SFN numbering information from signals received from the base station, together with reading other information required for communication with the base station, such as information regarding the particular frame format (including frame structure—i.e. structure at the sub-frame level). In certain systems, and again for example those using the UTRA and E-UTRA networks, the radio frame formats of different base stations are, in general, not synchronised with each other. Furthermore, although the frame lengths (i.e. durations) are typically the same, the frame structures of different base stations may be different (with structure varying with SFN in one format, and remaining the same, or varying in a different way with SFN in the different format of an adjacent base station or cell). The SFN may also be required by the UE for other purposes (i.e. other than to determine frame format) in order for the UE to communicate with the base station (for example by sending a RACH transmission). For example, a resource or resources allocated to the UE may be expressed in relation to the SFN (i.e. the resource may apply in some of the radio frames, but not others). In certain radio frames, the UE may not be required to receive any signals, and so it may go into sleep mode to conserve battery power. Thus, for a variety of reasons, the UE may require a knowledge of at least one or more least significant bits of the SFN (when expressed in binary form), or indeed the whole SFN, in order to operate appropriately when in communication with a base station.
Thus, in general, in a system using the UTRA or E-UTRA networks, at a particular time the current frames of two different base stations will have different SFNs, and the frame boundaries will be occurring at different times (i.e. the beginning of a frame in one format will occur at a different time to that of a frame in the second format). Clearly, this lack of synchronisation between base station's frame formats poses problems for handover of communications; the UE will be synchronised with the frame format of the base station with which it is currently communicating (the source base station), but cannot communicate with the target base station until it can synchronise with the frame format of that target.
In more detail, before handover, the UE normally measures the target cell. If this target cell becomes a good candidate, or indeed the best candidate, the UE reports this to the network (via the source cell, i.e. the current cell in which the UE is communicating with a respective base station). As an initial step, preceding the actual measurements, the UE searches for and detects the candidate/target cell using specific physical layer channels defined for this purpose (and known as SCH). As part of this search process, the UE is able to determine the boundaries of the radio frames in the target cell (i.e. the boundaries of the frames of the sequence of frames in the defined format for the base station of the target cell). A UE typically starts communication in an E-UTRA cell using the random access procedure, which involves an initial transmission on the Random Access Channel (RACH). E-UTRA uses a radio frame format in which radio frames have a duration of 10 ms, and are numbered by means of the System Frame Number (SFN). This SFN is indicated on the primary broadcast channel (P-BCH) (or simply the broadcast channel BCH), i.e. the SFN information of the particular base station is contained in signals transmitted from the base station on the P-BCH/BCH. To perform the initial transmission on the RACH, the UE needs to be aware of the SFN timing of the concerned cell (the target base station). This is needed for the following reasons:
Firstly, the UE needs the target SFN timing information to find the RACH slots (i.e. determine the time slots in the target base station's frame format in which the UE may transmit its RACH signal; if it does not transmit in the correct slot or slots, communication with the base station will not be set up). For the frequency division duplex (FDD) mode of operation, the interval between RACH slots in UTRA/E-UTRA formats can be as follows: 1, 2, 5, 10, or 20 ms. If the target cell is arranged to apply an interval of 20 ms (which means that a RACH slot occurs not in every frame, but in every other frame) the UE needs to know the least significant bit of the SFN to be able to find the RACH. In other words, if the UE knows from other frame format information provided to it in a handover signal, for example, that RACH slots occur only in evenly numbered frames, the UE then must be able to determine the least significant bit of the SFN in binary form of a particular frame in order to transmit a RACH signal at an appropriate time.
Secondly, the UE needs the target SFN timing information to know the time frequency resources when hopping is used for RACH. In certain systems frequency hopping techniques are used such that the RACH signal frequency hops in time according to a particular pattern. In UTRA/E-UTRA systems the RACH preamble frequency hopping period can either be 10 or 40 ms. Thus, in the former case the RACH frequency changes every frame, whilst in the latter case, the RACH frequency changes every four frames. If the target cell applies a 40 ms hopping period, the UE therefore needs to know the two least significant bits of the SFN to be able to determine the frames in which RACH frequency changes, so that it can correctly access the RACH (i.e. send a RACH signal, using the appropriate frequency, at the appropriate time).
The SFN may be needed for other reasons as well, or alternatively (e.g. where resource allocation is SFN dependent) as discussed above.
Thus, when a handover from one base station to another is required (or when communication between the UE and target base station is required for some other purpose) the UE, synchronised with the source base station, needs the SFN and timing information of the target so that it can initiate communication with the target.
One mechanism by which the UE can determine the SFN and timing information of the target cell is for the UE to read the PBCH or BCH of the target cell (base station). The UE can then implicitly determine the two least significant bits of the SFN from decoding the BCH, and can detect frame edges. The BCH is repeated every 40 ms, meaning that it takes on average 20 ms to receive the BCH, i.e. on average it will take 20 ms before the UE can determine the target SFN timing information it requires to begin communication with the target (by sending a RACH signal). BCH reading delay therefore increases the handover interruption time. Currently there are no other reasons for the UE to receive the BCH prior to accessing the target cell's RACH. All other information the UE requires to access the target cell is assumed to be semi-static, so it can be provided to the UE in the handover command that the target eNB generates and transfers to the UE via the source eNB, i.e. prior to the actual handover. This way, there is no need for the UE to read system information from the target cell.
It is an object of certain embodiments of the present invention to provide communication systems and methods which obviate or mitigate at least one of the problems associated with the prior art. It is an object of certain embodiments to provide communication systems and methods offering improved handover (e.g. faster and/or to reduce service interruption).